mahfuzmatyaacob

Hey guys,

Im working on a small project to linearize exponential decay (discharge voltage or maybe charging voltage). The circuit need to linearize it according to a math func (probably). Any idea u guys might wanna share?

Thanks!

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-Mahfuz-

Papabravo

Would you care to offer a precise definition of what it means to "linearize" an exponential waveform? It sounds like a really silly idea. Why would you want to do such a thing?

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mahfuzmatyaacob

It is a test fixture for embedded A/D. The discharge voltage is used as the input. But since A/D needs an increasing ramp voltage, the discharge is converted into linearized increasing ramp. (discharge region is used instead of charging because of the time constant)

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-Mahfuz-

ronsimpson

An RC causes the exponential decay.
Use a current source and capacitor.

mahfuzmatyaacob

Precisely.. But that would be just like creating a ramp circuit, not converting it to linearize output

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-Mahfuz-

k7elp60

As I remember if you take and op amp in the inverting mode, put a capacitor from the - input to ground and connect a resistor to a + supply the output from the op amp will be a linear increasing rampas the capacitor charges. If once the capacitor is charged and then the resistor lead that was connected to the + supply is grounded the output of the op amp would be a linear ramp going down. Perhaps some modifications of this circuit would solve the problem.

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mahfuzmatyaacob

I already have the circuitry for discharge voltage. The thing is how to convert each point of voltage discharge to an increasing ramp voltage. Im thinking of using programmable ALU (writting ln function since discharge is in exponential) to convert the voltage decay to a constant increasing voltage. Hows that?

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-Mahfuz-

kchriste

You haven't posted your circuit, so this idea may not be relevant. What about converting your single slope ADC to a dual-slope ADC so the non-linearity cancels out?

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Papabravo

Would a logarithmic amplifier do the job?

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mahfuzmatyaacob

Logarithmic amplifier? Well im not used to it so im know sure how that thing operates. Can the logarithmic amp be setted to a certain formula that enable the discharge to be linearized as in a factor of ln?

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-Mahfuz-

Papabravo

Yes. The output of a logarithmic amplifier is the logarithm of the input. So, says me, if you feed an exponential signal in you'll get the exponent out which is a linear function of the time t. N'est pas?

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mahfuzmatyaacob

Guess i should have use it then... Providing an exponential decay input to get the desired linearized output.. Logarithmic amp, sounds interesting.. WIll have a check on that device. Still need to figure out the design...

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-Mahfuz-

DigiTan

If you take a 555 timer and inject a constant current source into its its threshold and discharge pins, you will see linear ramp appear on that pin. The straightness of the ramp depends on the flatness of the current source's I-V characteristic.

I used the attached system in a ramp ADC running close to 44kHz. The ramp will appear on the collector of Q1. The trigger can be the output of another 555 if you want. I'd also adjust C1 and C2 to get right time constant.

The main limit is that it will only drive high-impedance loads. Our load was about 100kOhms. When we lowered it below 50k, the ramp seemed to destabilize.

Attached Images

ramp.gif (14.7 KB, 6 views)

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Roff

DigiTan, he has already made it clear that his goal is not to generate a linear ramp per se. His goal is to linearize the exponential discharge of a capacitor.

I agree that a log amp is the way to do it.

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Ron

mahfuzmatyaacob

Yeah, u got it right Roff... Anyway, Thanks DigiTan...

I do agree that a log amp is suitable, but can it be programmed to do certain formula conversion? I dont think so.. For example, the Q1 is the highest input value (exponentially decaying to 0V). So, the conversion need to be -ln (input_value/Q1).. This will generate a linearized waveform from an exponential decay input.. Each input_value is actually the value of the exponential decay voltage.

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-Mahfuz-